The invention relates to torque wrenches and more particularly to a light weight, self-contained, portable torque wrench wherein the torquing force is supplied by fluid power cylinders arranged such that equal, diametrically opposed forces are applied to the part being torqued without induced side loading.
Many current applications require high torquing forces to be applied with great precision, while simultaneously maintaining a precision alignment of the parts being joined. This is true, for example, in the assembly and repair of jet engines and other high speed machinery. It has been found that prior art torque wrenches, such as those discussed below, can not always meet these requirements. A typical prior art design utilizes an offset, single mechanical moment arm or single hydraulic ram assembly which has the tendency to cause side loading, especially at high torques. Such side loading can lead to bending or eccentricity of the parts being joined. Even a slight eccentricity, for example, in a turbine shaft, can cause unacceptable run-out with an attendant increase in engine vibration, leading to premature engine failure.
In the field of torque wrenches there is a wide selection of known wrenches designed for particular applications. Even in the somewhat narrower field of fluid power torque wrenches there is a great variety of designs. U.S. Pat. No. 4,137,800 to Austin, for example, discloses a torque wrench in which a hydraulic cylinder forces a slide block against a single torque arm causing the rotation of a ratchet mechanism. U.S. Pat. No. 3,686,983 to Flagge discloses a torque-applying device wherein the torquing force is supplied by an hydraulic motor. One example of a specialized torque wrench available in the prior art is disclosed in U.S. Pat. No. 4,838,130 to Snyder which describes a hydraulically actuated power wrench specifically adapted for torquing one of a plurality of adjacently spaced bolts. The Snyder '130 wrench comprises a hydraulic cylinder which acts on a pivotable lever to turn a bolt. The reaction force due to the application of torque is borne by an adjacent bolt through a specially shaped anchoring ring. U.S. Pat. Nos. 3,868,872 and 4,706,527 illustrate further examples of torque wrenches where the torquing force is provided by a hydraulic cylinder acting through a lever arm.
U.S. Pat. No. 2,961,904 discloses a hydraulically actuated wrench which attempts to address the failings of the prior art by applying a balanced torquing force. The subject wrench is provided with a central drive member having a ratcheted opening for engaging a nut. The drive member also has three arms equally spaced at 120 degree intervals extending radially from the drive member. Three pairs of fixed, opposed hydraulic cylinders act on these arms to provide the torquing force. The cylinders are equally spaced about the drive member in order to exert a balanced force on the work piece. The cylinders are arranged in opposed pairs in order to enable the wrench to operate in either direction, i.e., three cylinders exert a clockwise force and three exert a counter-clockwise force.
While the patented wrench disclosed in the '904 patent is an improvement over the previously available wrenches in that it applies a generally balanced torquing force, its design is still not satisfactory for all applications. The triangular arrangement of the cylinders, while generally providing balanced forces, is inherently somewhat unstable due to the fact that each cylinder, acting on the work piece through the drive member, is not directly opposed by an equal reaction force. Therefore, the possibility exists for side loading to occur, causing the piston to creep on the arm of the drive member. Such creep would change the effective length of the moment arm and thus cause inaccurate torque readings.
Another disadvantage of a design using opposed cylinders for supplying torquing forces in two directions is the precision machining which is required to fabricate such a wrench. In order for the torque readings to be accurate in both directions, the center lines of the opposed cylinders must match exactly. If the center lines do not match, there will be different effective moment arms depending on which cylinder is acting. It is expensive and time consuming to precision line bore the required seat for the opposed cylinders. In large wrenches, requiring large diameter bores, the tolerance requirements alone may be sufficient to cause a center line mismatch.
In addition, paired cylinders such as are found in the '904 wrench require two travel limit valves to prevent piston over travel. This extra hydraulic valving and attendant piping must be placed on the outside of the wrench as a result of design size considerations. The valving and piping is therefore subject to damage and leaking when the wrench is used under normal field conditions.
A further disadvantage of the prior art wrenches discussed above is that in order to achieve a large torque capacity, the weight and size of these wrenches are such that they are bulky and cumbersome to use. This is especially true of the wrench of the '904 patent because six cylinders are included while only three at a time are used for a particular torquing operation. This greatly increases the weight of the wrench. Portability is a very important feature in torque wrenches as described herein, especially if the wrench is to be successfully utilized at remote field installations as is required in the offshore oil industry and in many military applications.